While you've got the case off, check ALL your internal boards, make sure they're all seated properly - RAN chips, Video board, network card, soundcard, modem etc.

 
First thing I'd do is reseat the RAM and then test it.
DDR3 is the most common source of stability issues with newer systems.
Even if it's new, don't assume it's good.
 
PS can also cause spontaneous reboots...
If it happens only when putting the system under heavy loads, that would point to the PS.